linux-kernel - Re: [PATCH] thermal: sysfs: Perform bounds check when storing thermal states

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Date:   Tue, 5 Jul 2022 23:02:50 +0200
From:   Varad Gautam <varadgautam@...gle.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Amit Kucheria <amitk@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>, linux-pm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] thermal: sysfs: Perform bounds check when storing thermal states

On Tue, Jul 5, 2022 at 6:18 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Tue, Jul 05, 2022 at 03:00:02PM +0000, Varad Gautam wrote:
> > Check that a user-provided thermal state is within the maximum
> > thermal states supported by a given driver before attempting to
> > apply it. This prevents a subsequent OOB access in
> > thermal_cooling_device_stats_update() while performing
> > state-transition accounting on drivers that do not have this check
> > in their set_cur_state() handle.
> >
> > Signed-off-by: Varad Gautam <varadgautam@...gle.com>
> > Cc: stable@...r.kernel.org
> > ---
> >  drivers/thermal/thermal_sysfs.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/thermal/thermal_sysfs.c b/drivers/thermal/thermal_sysfs.c
> > index 1c4aac8464a7..0c6b0223b133 100644
> > --- a/drivers/thermal/thermal_sysfs.c
> > +++ b/drivers/thermal/thermal_sysfs.c
> > @@ -607,7 +607,7 @@ cur_state_store(struct device *dev, struct device_attribute *attr,
> >               const char *buf, size_t count)
> >  {
> >       struct thermal_cooling_device *cdev = to_cooling_device(dev);
> > -     unsigned long state;
> > +     unsigned long state, max_state;
> >       int result;
> >
> >       if (sscanf(buf, "%ld\n", &state) != 1)
> > @@ -618,10 +618,20 @@ cur_state_store(struct device *dev, struct device_attribute *attr,
> >
> >       mutex_lock(&cdev->lock);
> >
> > +     result = cdev->ops->get_max_state(cdev, &max_state);
> > +     if (result)
> > +             goto unlock;
> > +
> > +     if (state > max_state) {
> > +             result = -EINVAL;
> > +             goto unlock;
> > +     }
> > +
> >       result = cdev->ops->set_cur_state(cdev, state);
>
> Why doesn't set_cur_state() check the max state before setting it?  Why
> are the callers forced to always check it before?  That feels wrong...
>

The problem lies in thermal_cooling_device_stats_update(), not set_cur_state().

If ->set_cur_state() doesn't error out on invalid state,
thermal_cooling_device_stats_update() does a:

stats->trans_table[stats->state * stats->max_states + new_state]++;

stats->trans_table reserves space depending on max_states, but we'd end up
reading/writing outside it. cur_state_store() can prevent this regardless of
the driver's ->set_cur_state() implementation.

Regards,
Varad

> thanks,
>
> greg k-h